The objective of the proposed research is to learn more about the mechanisms responsible for asymmetries in the vestibuloocular reflex (VOR) after unilateral vestibular lesions. The research strategy is to analyze responses of the system through eye-movement studies with rotational and translational stimuli, single-unit recordings, galvanic labyrinthine polarizations, and control systems modeling. Responses are investigated before and after vestibular lesions (canal plugging or labyrinthectomy). The research is conducted in squirrel monkeys and in chinchillas. The studies will involve a three-dimensional analysis of the angular VOR before and after vestibular lesions. The dynamics of the vertical and torsional responses will be examined. The contributions of excitatory and inhibitory signals in determining the alignment of the angular VOR after unilateral vestibular lesions will be defined. The coordinate frame of the angular VOR will be measured before and after plugging of selected canals. The neural mechanisms underlying signal processing in the angular VOR will be investigated. The signals carried by different physiological groups of vestibular-nerve afferents will be defined. The responses of central vestibular neurons involved in control of the angular VOR will be studied for higher frequency and acceleration rotational stimuli that have been shown to evoke asymmetries in the reflex after unilateral vestibular lesions. The linear VOR evoked by interaural translations (lateral LVOR) will be studied before and after unilateral labyrinthectomy. The responses of otolith afferents to a range of linear accelerations measured, and their role in the control of the lateral LVOR will be defined. The interactions of rotational and translational VORs will be investigated. This research has importance for defining the basic mechanisms of vestibuloocular signal processing and for improving the diagnosis and treatment of vestibular disorders in humans.